ANIMAL LOCOMOTION. 



extended. The rapidity with which the wing of the bird is 

 extended and flexed is in some instances exceedingly great.; 

 so great, in fact, that it escapes the eye of the ordinary observer. 

 The speed with which the wing darts in and out in flexion 

 and extension would be quite inexplicable, but for a know- 

 ledge of the fact that the difi'erent portions of the pinion form 

 angles with each other, these angles being instantly increased 

 or diminished by the slightest quiver of the muscular and 

 fibro-elastic systems. If we take into account the fact that 

 the wing' of the bird is recovered or flexed by the combined 

 action of voluntary muscles and elastic ligaments ; that it is 

 elevated to a considerable extent by voluntary muscular effort ; 

 and that it is extended and depressed entirely by muscular 

 exertion, we shall have difficulty in avoiding the conclusion 

 that the wing is thoroughly under the control of the muscular 

 system, not only in flexion and extension, but also throughout 

 the entire down and up strokes. 



An arrangement in every respect analogous to that described 

 in the bird is found in the wing of the bat, the covering or 

 web of the wing in this instance forming the principal elastic 

 ligament (fig. 17, p. 36). 



Power of the Wing — to what owing. — The shape and power 

 of the pinion depend upon one of three circumstances — to 

 wit, the length of the humerus,^ the length of the cubitus or 

 forearm, and the length of the primary feathers. In the 

 swallow the humerus, and in the humming-bird the cubitus, 

 is very short, the primaries being very long ; whereas in the 

 albatross the humerus or arm-bone is long and the primaries 

 short. When one of these conditions is fulfilled, the pinion 

 is usually greatly elongated and scythe-like (fig. 62, p. 137) 

 — an arrangement which enables the bird to keep on the 

 wing for immense periods with comparatively little exertion, 

 and to wheel, turn, and glide about with exceeding ease and 

 grace. When the wing is truncated and rounded (fig. 96, p. 



1 The humenis varies extremely in length, being very short in the swal- 

 low, of moderate length in the gallinaceous birds, longer in the crows, very 

 long in tlie gannets, and unusually elongated in the albatross. In tlie golden 

 eagle it is also seen to be of great length." — Macgillivray's Britisli Birds, 

 vol. i. p. 30. 



